1. Field of the Invention
This invention relates generally to the production of polyhydric compounds and, more specifically, to an improved process for producing polyhydric compounds by hydrolysis of carboxylate esters.
2. Description of the Prior Art
Diols and triols are known compounds, many of which are produced commerically in substantial quantities. Thus, ethylene glycol and 1,2-propylene glycol (hereinafter referred to as "propylene glycol") are chemicals of acknowledged commercial importance. Ethylene glycol, for example, is used widely in the preparation of anti-freeze compositions and in the manufacture of polyester fibers. Propylene glycol finds widespread use in the manufacture of antifreeze, perfumes and solvents. Ethylene glycol manufacturing processes of commercial interest have generally been based upon ethylene oxide as a raw material. Recently, however, processes have been developed which make it possible to produce diols such as ethylene glycol and propylene glycol without the necessity for the intermediate manufacture of the epoxide. These processes employ the liquid phase reaction of the appropriate olefin, a carboxylic acid, and molecular oxygen in the presence of a catalyst to produce carboxylic acid esters of the glycol. Processes of this type are disclosed in U.S. Pat. Nos. 3,262,969; 3,668,239; 3,689,535; 3,715,389; 3,743,672; 3,770,813; 3,778,468; and 3,872,164, and in Canadian Pat. No. 888,749. The glycol can be liberated by hydrolysis of the carboxylate esters produced in such processes, and the glycol then recovered from the hydrolysis effluent. Thus, as is disclosed in U.S. Pat. Nos. 3,809,724 and 3,859,368 the carboxylate esters can be hydrolyzed with water in the presence of an acidic ion exchange resin catalyst or thermally.
The hydrolysis effluent, which normally contains the glycol, water, carboxylic acid, unreacted carboxylate esters and by-products, is then distilled generally in two successive steps, to remove water and carboxylic acid as overhead products, and the essentially water- and carboxylic acid-free liquid so produced is then treated for recovery of the desired glycol. The water so removed is generally recycled to the hydrolysis reactor and the carboxylic acid is desirably recycled to the oxidation step for use in forming additional carboxylic acid esters. Particularly efficient methods for separation of the glycol from any unconverted esters present in the liquid so treated are the azeotropic distillation processes disclosed in U.S. Pat. Nos. 3,809,724, 3,859,368 and 4,021,311, and co-pending application Ser. No. 612,825 (filed Sept. 12, 1975), now U.S. Pat. No. 4,057,471 and Ser. No. 776,392 (filed Mar. 10, 1977), now abandoned the disclosures of each of which are hereby incorporated by reference. According to these azeotropic distillation processes, a glycol-azeotroping agent (e.g., diethylbenzene, trimethylbenzene and the like) is employed to facilitate recovery of the desired glycol as overhead, which can then be treated for separation of the glycol from the glycol-azeotroping agent. The bottoms from the azeotropic distillation column contain unhydrolyzed carboxylate esters of the glycol and can be advantageously recycled to the hydrolysis reactor for formation of additional glycol therefrom. However, these bottoms can also contain small amounts of the glycol-azeotroping agent, and since the agent passes through the hydrolysis reactor substantially unaffected, glycol-azeotroping agent is present in the hydrolysis effluent. As a consequence, the subsequent successive distillation of the hydrolysis effluent to remove water and carboxylic acid results in the passage of significant amounts of the glycol-azeotroping agent into the carboxylic acid overheads product.
Since recycle to the oxidation step of a carboxylic acid stream containing substantial amounts of the glycol-azeotroping agent is highly undesirable due to the tendency of these materials to form tar-like substances during the oxidation and the attendant equipment fouling problems, the need exists to minimize the concentrations of azeotroping agent passing into the carboxylic acid overheads.